The present invention relates to methods and apparatus for separation by thermally cycled sorption and desorption.
Separation of fluid components from fluid mixtures has been a topic of great scientific and economic interest for more than 100 years. This invention concerns methods and apparatus for separating fluid components from fluid mixtures by thermally cycled sorption and desorption. Our experimental work, and much of the following descriptions, concern separating hydrogen gas. In its broader aspects, however, this invention is applicable to any fluid, either gaseous or liquid including supercritical fluids.
Purified hydrogen has long been and continues to be used in a variety of industrial processes. For. example, petroleum refineries are using increasing quantities of hydrogen to meet regulatory requirements on diesel, gasoline, and other petroleum products. Hydrogen-based treating processes are expected to grow substantially because fuel regulations in North America, Europe, and other regions are becoming increasing stringent. For example, the sulfur levels in U.S. diesel fuels must decrease from the current level of 250 ppm to 15 ppm by 2007. While several options exist for lowering sulfur levels, all of commercially available processes require a hydrogen input stream.
Another major use of hydrogen is in upgrading crude oil to make gasoline. To meet the world""s increasing demand for gasoline, it has been necessary to develop poorer grades of crude oil that are denser and require hydrogenation for upgrading to gasoline.
Additionally, for more than 10 years there have been intense research and development efforts directed toward hydrogen as a clean power source for fuel cells. Compared to conventional power systems, hydrogen-powered fuel cells are more energy efficient, more robust, and less polluting. Fuel cells can totally eliminate ozone and nitrogen oxides, the most noxious precusors of smog. However, problems such as excessive cost, equipment size, and process complexity have prevented hydrogen-based fuel cell technology from replacing most conventional power sources.
The present invention provides apparatus and methods for separating fluids. The invention can be used, for example, to purify hydrogen formed in a steam-reforming reaction (typically a gas containing hydrogen, carbon monoxide and carbon dioxide). Compared to conventional fluid separation technology, many of the configurations and procedures of this invention are relatively simple, scaleable over a broad range, including small, and are amenable to cost-effective mass production.
In a first aspect, the invention provides a method of separating a fluid component from a fluid mixture including at least two steps. In the first step, a fluid mixture passes into a flow channel at a first temperature. The flow channel comprises a sorbent within the channel, and flow through the flow channel is constrained such that in at least one cross-sectional area of the channel, the height of the flow channel is 1 cm or less. Heat from the sorbent is transferred to a microchannel heat exchanger. The fluid mixture contacts the sorbent without passing through a contactor. Then, in a second step, energy is added and the temperature of the sorbent is increased. A fluid component is desorbed from the sorbent at a second temperature and a fluid component that was sorbed in the first step is obtained. The second temperature is higher than the first temperature.
In a second aspect, the invention provides another method of separating a fluid component from a fluid mixture that includes at least two steps. In a first step, a gas mixture passes into a flow channel at a first temperature. The flow channel comprises a sorbent within the channel, and flow through the channel is constrained such that in at least one cross-sectional area of the channel, the height of the flow channel is 1 cm or less. Then, in a second step, energy from an energy source is added and the temperature of the sorbent is increased. A fluid component is desorbed at a second temperature and a fluid component that was sorbed in the first step is obtained. The second temperature is higher than the first temperature. The first and second steps, combined, for a non-condensed fluid mixture (i.e., a gaseous or supercritical fluid) take 10 seconds or less and wherein at least 20% of the gaseous component sorbed in the first step is desorbed from the sorbent; or for a liquid mixture take 1000 seconds or less and wherein at least 20% of the fluid component sorbed in the first step is desorbed from the sorbent.
In a third aspect, the invention provides another method for separating a fluid component from a fluid mixture. In this method, a fluid mixture passes into a first sorption region at a first temperature and first pressure, wherein the first sorption region comprises a first sorbent and wherein the temperature and pressure in the first sorption region are selected to favor sorption of the fluid component into the first sorbent in the first sorption region. Heat from the first sorption region is transferred into a microchannel heat exchanger. A fluid component from said fluid mixture is selectively sorbed, thus resulting in a sorbed component in the first sorbent and a fluid mixture that is relatively depleted in said component. The relatively component-depleted fluid mixture is passed into a second sorption region at a second temperature and second pressure, wherein the second sorption region comprises a second sorbent and wherein the temperature and pressure in the second sorption region are selected to favor sorption of the fluid component into the sorbent in the second sorption region. Heat transfers from the second sorption region into a microchannel heat exchanger. The fluid component is selectively sorbed from said relatively component-depleted fluid mixture thus resulting in sorbed component in the second sorbent and a relatively more component-depleted gas mixture. The second temperature is different than the first temperature. Heat is added to the first sorbent, through a distance of about 1 cm or less to substantially the entire first sorbent, to raise the first sorbent to a third temperature and the component is desorbed from the first sorbent. Heat is added to the second sorbent, through a distance of about 1 cm or less to substantially the entire second sorbent, to raise the second sorbent to a fourth temperature and the component is desorbed from the second sorbent; and the component desorbed from the first and second sorbents is obtained.
In a fourth aspect, the invention provides a fluid separation apparatus that includes: a flow channel comprising a porous sorbent, the flow channel having at least one dimension of 1 cm or less, wherein, in at least one cross-section of the flow channel the porous sorbent occupies at least 90% of the cross-sectional area; and a microchannel heat exchanger in thermal contact with the flow channel. The invention also provides a use of this apparatus to purify a fluid component from a fluid mixture.
In a fifth aspect, the invention provides a fluid separation apparatus including: a first array of flow channels, a second array of flow channels, at least one fluid conduit connecting the outlet of the first array to the inlet of the second array; and a valve capable of controlling the flow through the fluid conduit. The first array of flow channels includes: at least two flow channels, each of which includes an inlet, an outlet and a sorbent disposed between the inlet and the outlet. Each of the at least two flow channels are in thermal contact with a microchannel heat exchanger and have at least one dimension of 1 cm or less. This dimension is in a direction toward a microchannel heat exchanger. The first array also includes at least one array inlet and at least one array outlet. The second array of flow channels includes: at least two flow channels, each of which includes an inlet, an outlet and a sorbent disposed between the inlet and the outlet. Each of the at least two flow channels are in thermal contact with a microchannel heat exchanger and have at least one dimension of 1 cm or less. This dimension is in a direction toward a microchannel heat exchanger. The second array also includes at least one array inlet and at least one array outlet. The invention also includes a method of using this apparatus in which a fluid component is sorbed in the first array, and, simultaneously, a fluid component in the second array is desorbed.
The low pressure changes involved in the temperature swing sorption (TSS) process of the invention allow very thin metal shim and foil construction, hence, allowing very low metal mass and therefore very fast cycle times. Thin-walled construction also allows high surface area per volume of TSS device, thereby producing high rates of productivity per unit volume of equipment, thereby providing a high productivity rate needed for industrial scale processing. Also, an important facet of the invention is that the high surface area per unit volume (SA/V) feature of the hardware allows the sorbent to be deposited within the device in a high surface area, thin film fashion. Hence, the sorbed species, once formed on the surface of the sorbent, does not need to migrate far to fully load the sorbent internal solid volume, which, in comparison, would be a slow process in thick sorbent beds containing coarse particles to reduce pressure drop across the bed. A benefit to the inventive TSS design is that, since only very thin sorbent layers are needed, and can be cycled rapidly, sorbent material normally considered too costly, such as Pd, can be used.
Numerous advantages are provided by various embodiments of the present invention including: reduced cost, reduced volume of separation hardware, durability, stability, separation speed, ability to separate large volumes of fluid components with a small volume of equipment, improved energy efficiency and reduced cost relative to packed bed or membrane technology.
The invention includes apparatus having any of the configurations indicated in the figures. However, these specific configurations are not the only means to carry out the invention and, therefore, should not be interpreted as limiting the inventive apparatus or methods. The invention also includes methods in which a fluid mixture passes through any of the illustrated apparatus. For example, with reference to FIG. 5a, the invention includes a method in which a fluid mixture flows through a flow distribution sheet and the distributed flow passes into a sorbent-containing compartment.
xe2x80x9cHardware volumexe2x80x9d means the external volume of the separator apparatus including the sum of all parts if the apparatus is not integrated in a single unit.
xe2x80x9cInternal surfacexe2x80x9d refers to any surface in the interior of the flow channel that is exposed to flowing fluid. Internal surface may be measured by appropriate techniques such as optical measurement or N2 adsorption.
xe2x80x9cSorption/desorptionxe2x80x9d refers to the total amount of gas taken in without regard to the mechanism by which the fluid is taken in. In other words, xe2x80x9csorptionxe2x80x9d is the sum of adsorption and absorption.
The term xe2x80x9cfluid mixturexe2x80x9d means a fluid mixture containing between 1 and 999,999 parts per million (ppm) of a first component and at least one ppm of a component other than the first component.
The term xe2x80x9ccomponentxe2x80x9d refers to a molecular species. It should be understood that any of the methods described herein could separate (for example, sorb) more than one component; but the sorption step selectively partitions components, that is, a sorption step either increases or decreases the relative amount of a selected component in the gas mixture.
Occasionally, the specification uses the term xe2x80x9csolute.xe2x80x9d This term means component. The term xe2x80x9csolutexe2x80x9d does not require that component to be present in less than 50% by volume or mass.
The term xe2x80x9chydrogenxe2x80x9d as it is used throughout the specification includes hydrogen and all its isotopes.
The term xe2x80x9cobtainingxe2x80x9d means that the component is recovered either for storage or for use in a subsequent chemical process such as combustion, fuel cell operation, chemical synthesis, etc. The term xe2x80x9cobtainingxe2x80x9d does not mean, however, that the component is used simply as a refrigerant.
The term xe2x80x9cheat exchangerxe2x80x9d means a component, or combination of components, that is capable of adding and removing heat. Preferred examples of heat exchangers include microchannels that can be switched from hot to cold fluids, electrical resistors in combination with a heat sink, and thermoelectric materials.
xe2x80x9cTSAxe2x80x9d is thermal swing adsorption.
A xe2x80x9cporous contactorxe2x80x9d is a porous or perforated material through which flow occurs to reach a sorbent.